Currently, in order to be able to generate a low-frequency (LF) shield around or inside a vehicle from one or more coordinated LF antennas, the transmitters must work with a control signal with a variable duty cycle centered around the resonant frequency of the antenna in question. The LF transmission system filters this control signal with a band-pass filter so as to extract the fundamental harmonic from it and to generate a current necessary for the creation of a magnetic field.
Depending on the passband of the band-pass filter used and on the type of control signal, the harmonics at transmission are filtered more or less. In an unfavorable case, an asymmetric control signal linked with a large passband will allow unwanted harmonics and hence noise to pass.